Various systems have been proposed which employ inductive coupling to transmit electromagnetic power, data, and/or control signals between downhole equipment (such as pressure and temperature sensors, perforating guns, and valves) and surface equipment. In such systems, electric signals are induced in a first downhole coil from a second downhole coil adjacent to the first coil. Such inductive coupling desirably eliminates the need to mechanically connect the elements on which the coils are mounted, and thus greatly simplifies the handling of downhole equipment in preparation for (and during) drilling, logging, and producing operations.
It would be desirable to design such inductive coupling transmission systems to have a minimum number of downhole components, to have a high degree of reliability when installed in a well, and to be able to communicate power and data signals across mechanical pressure boundaries, with pressure differentials of up to many thousands of pounds per square inch, without the need for mechanical penetration. It would also be desirable to design such inductive coupling transmission systems so that the passive components (cable, coil windings, etc.) may be permanently installed in a well, while the active components (downhole sensor, transmitter, etc.) which more frequently fail may be installed and retrieved by standard wireline techniques. It would also be desirable to design such inductive coupling transmission systems so that a downhole measuring system may be added to an existing downhole safety valve installation (such as that described in U. S. Pat. No. 4,852,648, issued Aug. 1, 1989, to Akkerman, et al.) with a minimum of added downhole components, and without the need for a tubing run. Furthermore, it would be desirable to design a downhole measuring system that consumes a minimum of power and is compatible with inherently inefficient inductive coupling transmission systems for powering a safety valve.
However, until the present invention, it had not been known how to design inductive coupling transmission systems to have downhole measuring capability, and to embody the above-mentioned desirable features.